The present invention relates to the field of analog signal processing. In particular, this invention relates to an apparatus and a method for providing a current exponentially proportional to voltage and directly proportional to temperature. Such an apparatus and method can be used in an automatic gain control system.
Automatic gain control ("AGC") is a process or means by which gain is adjusted automatically in a specified manner as a function of input or other parameters. It is well known that the gain of variable gain amplifiers can be so adjusted or varied. Automatic gain control can be implemented in a known way by means of an electrical feedback loop from the output of an amplifier to the gain control input of the amplifier.
A known variable gain amplifier includes two parts: (1) an exponential converter that converts a control voltage to a current that is a function of the logarithm of the control voltage, and (2) an amplifier that has a gain that is a function of the current out of the exponential converter. A monolithic analog exponential voltage-to-current converter based on the exponential characteristic of bipolar transistors is described in an article entitled A Monolithic Analog Exponential Converter by J.H. Huijsing and J. Anne van Steenwijk in IEEE Journal of Solid-State Circuits, Vol. SC-15, No. 2, pp. 162-68 (April 1980).
Both of the above-mentioned parts of a variable gain amplifier (i.e., the exponential converter and the amplifier) have temperature coefficients that alone are undesirable.